1. Field of the Invention
This invention relates to icebreaking ships for opening channels through ice fields so that merchant ships can pass relatively unhindered through the ice fields. These icebreaking ships often need to turn around within a limited space, and therefore, the hulls of the ships need to be designed to allow for narrow radius turns as well as for efficient breaking and clearing of the ice to form substantially ice-free channels to allow for passage of the merchant ships. Such a hull can be designed with two sets of icebreaking portions disposed at both sides of the forward portion of the hull, in the vicinity of the waterline. The width of the hull at the two ice-breaking portions should preferably not exceed the width of the ship. In addition, between the two icebreaking portions, on each side of the hull, there is preferably a recessed area which has a profile that substantially matches the radius of curvature of the inside turning circle of the ship at the zenithal line of the turning circle.
2. Background Information
It is known that, in order to provide the most ice-free channel for passage of merchant ships through ice fields, the hull of an icebreaking ship should be designed to not only break up the ice, but to also push the ice out of the channel, for example, by pushing the ice outwardly below the ice boundaries of the channel. The broken ice therefore does not fill the opened channel, and therefore essentially does not interfere with the propellers of any ships passing through the opened channel.
Known icebreaking ships, for example, those disclosed in U.S. Pat. No. 4,831,951, have both the first and the second icebreaking portions project laterally beyond the width of the hull itself. Accordingly, these icebreaking portions form beak-like, or bay window-like projections attached to the ship's skin. The second icebreaking portion of the ship disclosed in U.S. Pat. No. 4,831,951, is located above the water line, while the recessed areas between the icebreaking portions are approximately equal to the width of the ship. This design, of course, allows the icebreaker to turn with a relatively small radius of turning circle, but the optimum ratio between the width of the ice-free channel that is broken by the ship and the width of the ship itself, is not achieved. This optimum ratio is not achieved because, as mentioned above, the outer limits of the icebreaking parts, which outer limits naturally determine the width of the ice-free channel being made, project beyond the width of the ship. In many cases, however, it is desirable in terms of the weight, propulsion force and stability of the ship for the hull to have the greatest possible width in relation to the width of the ice-free channel that the hull is forming, i.e. the width of the ship and the width of the ice-free channel should be as equal as possible.
Another known icebreaking ship is disclosed in German Laid Open Patent No. 2246173. This German Patent shows only a first icebreaking portion which has a lateral limitation that is flush with the external contour of the hull. There is also a waistline-like indentation into the hull behind the ice-breaking portion. However, this indentation is not an indentation between two separate icebreaking portions located one behind the other, but is an indentation behind a single icebreaking portion. This indentation is not intended to take advantage of the maximum width of the ship, but is used only for the most ice-free possible exhaust of compressed air from a duct system that is a part of a ramming or tamping system.